1. Field to Which the Invention Relates
This invention relates to a pipe laying vessel having a vertical pipe storage reel. In particular the present invention is directed to a water borne vessel that is self-propelled and directed for laying pipe at the bottom of a body of water. The vertical storage reel includes pipe that is spooled onto the reel for continuous pipe laying. The pipe is unspooled from the reel and directed towards the seabed where it subsequently may or may not be buried.
2. The Known Prior Art
Pipelines are laid offshore on a seabed for the purposes of transporting oil and gas from or between offshore platforms embodying subsea wells to onshore facilities. Such offshore subsea pipelines are common, for instance, in the Gulf of Mexico but are not limited to that geographic area. In the past, such pipelines have been installed solely to transport oil and gas but recently such subsea lines have been installed to transport other products such as water or slurries of a variety of materials. The traditional method that had been in use for many years to install a subsea pipeline is by the method often referred to as "stove-piping". With this method individual joints of pipe are transported to pipelaying vessels offshore where they are joined by welding prior to their descent to lay on the sea bottom. Each length of pipe may be as long as 80 feet but before any of the pipeline is paid out the descent of the pipeline to the bottom is not continuous because each new pipe length has to be welded onto the previous pipe length before it can pass from the pipe laying vessel. It is obvious that the stove-piping method is cumbersome, time consuming and unwieldly and until the advent of the vertical storage reel, the costs for pipelaying projects were extremely high due to the premium rates demanded by offshore personnel. Because of the slow progress of pipelaying in accordance with prior art methods, and the uncertainties that weather conditions would not hold for any extended period of time rendered the entire pipe laying operation subject to being achieveable only under the most harsh and unsafe conditions. The alternative was to shut down to await more acceptable weather but continue to endure huge fixed costs in personnel and equipment.
Additional problems found in prior art pipelaying vessels that produce pipelaying at low departure angles. Such laying of pipelines was realized by means of pontoons called "stingers" that support the pipeline to a required departure angle. These low departure angles are a limiting characteristic of conventional methods. Increasing the departure angle of the pipeline significantly reduces the amount of tension to maintain a desired geometry for a prescribed water depth and pipeline weight.
An entirely new pipelaying vessel concept arose with the prospects of joining the pipes onshore and storing them on a horizontally placed reel on a floating vessel. This format significantly reduced offshore pipeline deployment time and therefore costs. However, low departure angles still limited the maximum water depth and in addition, as the relatively small size of the pipeline storage reel permanently deformed the pipeline, straightening of the pipeline before its deployment became necessary.
The deformation inherent in the horizontal pipeline storage reel was the problem because the pipeline bent about a vertical plane around the horizontally placed storage reel and during the laying procedure as bending occurred around the horizontal plane of the pipeline biaxial bending was induced. This biaxial bending was difficult to control or eliminate from the line.
Well over a decade ago efforts were made to resolve some of these problems and build the first dynamically positioned reel ship using a vertical storage drum upon which the pipe was spooled and which therefore eliminated the biaxial bending and significantly increased the departure angle of the pipeline. A number of U.S. patents have been directed to this concept and include amongst others, U.S. Pat. Nos. 4,345,855, 4,340,322, 4,297,054, 4,269,540 and 4,230,421.
The prior art approach as embodied in the foregoing patents included a vertical reel pipelaying ship in which a large reel of spooled continuous pipe was located amidships and a stern ramp with an adjustment between 20.degree. to 60.degree. receives the pipe as it is unspooled. The ramp included a number of supporting elements in order to handle the pipeline as it is unspooled from the vertical reel.
An aligner was included in the ramp of the prior art vertical reel pipelaying vessel and the aligner included a series of closely spaced hourglass rollers or a conveyor-type belt to support the pipeline. The curvature of the aligner was slightly larger than the radius of the reel permanently deforming the pipeline. The aligner also provided a transition for the pipeline between the reel and the ramp and as its name implies, it also permitted the alignment of the pipeline to the vertical reel during the spooling operation.
The ramp also includes a straightener that consists of a series of tractor-type tracks supporting and forcing the pipeline, during lay into a reverse curvature. The assembly consists of two bottom tracks and another top track, bending the pipeline at three points. The reverse radius of curvature is forced upon the line by the action of the top track bearing between both bottom tracks. The aligner feeds the oncoming pipeline at a particular angle on top of the first bottom track that is set slightly higher than the second bottom track. The top track then bends the pipeline to the required reverse radius of curvature. Thereafter, the pipeline should be straight and parallel to the ramp.
A tensioner is also a required element of this prior art ramp, which as in all traditionally designed offshore pipelaying vessels provides all the required laying tension to the pipeline to permit the handling of the pipe by the pipelaying vessel during spooling and laying operations. The tensioner provides some of the tension on the line with the reel providing the required difference. The tensioner also shares the lower track with the straightener. A second adjustable top track presses on the line providing enough compression to develop the necessary friction between the line in the tracks. This friction translates into tension on the line.
All offshore pipelay operations involve abandoning and recovering the pipeline using a holding or clamping mechanism. For reel pipe laying, this mechanism is generally referred to in the art as an A&R clamp that performs the operations of clamping the pipeline for installation of the supporting head. The A&R clamp is typically a device comprising a series of inflatable bags designed for each pipeline size inside rigid half shells. Pressurizing the bags compress the pipeline and clamps it to the support structure.
The last structure supporting the pipeline before its entry into the water is a stern roller. Its function is to restrain the pipeline from jumping out of the ramp and to provide a reference point for laying the pipeline. The pipelay operator on the vessel maintains the pipeline at a constant height of 6 inches above the stern roller. This position as well as the known ramp angle permits the definition of the geometric characteristics of the suspended pipeline. Therefore tension is not a parameter that the operator controls. Rather it is a consequence of the geometry of the system. This known geometry permits the calculation of structural parameters therefore controlling the ideal stress level of the pipeline.